Electric double layer capacitors are electricity storage devices using electrostatic capacitance of electric double layers obtained by adsorption/desorption of electrolyte ions at the interfaces between a pair of positive/negative polarizing electrodes and electrolyte solution.
Applications of electric double layer capacitors are widely used from small capacitance devices (such as for memory back), to medium and large capacitance devices (such as for auxiliary power supplies for electric vehicles (EV), solar cells and wind power generations, and for voltage dip compensators). In recent years, for vehicles or construction equipment, further reducing fuel consumption by using regenerative energy has been sought; this is why electric double layer capacitors are considered realizable devices for achieving rapid charge/discharge rates.
Electric double layer capacitors' features to be evaluated are, for example, their electrostatic capacitance (C), working voltage (V), energy density (E), internal resistance, and useful life. For electric double layer capacitors, improvement of each feature is important; however, recently, improvement of their weak point, i.e. energy density, and further extension of their strong point, i.e. useful life (i.e. improvement of the long-term reliability) have been required for the applications of the above mentioned small, medium and large capacitance devices.
Patent Document 1 provides polarizing electrode materials containing a tungsten oxide powder, and an electric double layer capacitor using the materials; the electric double layer capacitor using the polarizing electrode materials has high energy density and low degradation of electrostatic capacitance and resistance with passage of time, i.e. excellent long-term reliability.
In addition, the polarizing electrode materials for the electric double layer capacitor are bonded to a current collector with a conductive adhesive based on water glass.
In Patent Document 1, tungsten oxide contained in the polarizing electrode materials enabled the electric double layer capacitor to have high energy density and low degradation of electrostatic capacitance and resistance with passage of time, i.e. excellent long-term reliability. Although it is not clear why good results were achieved by adding tungsten oxide to the polarizing electrode materials, the following mechanism is considered to exist: Inside an electric double layer capacitor, there is residual water in electrolyte solution, and water that entered the inside upon assembling the electric double layer capacitor. There is also physically adsorbed water or chemically-bonded adsorbed water on the surfaces and inside of activated carbon, an auxiliary conducting agent and a binder used to form the electrode layer, and a separator, etc. This water is separated as water when repeatedly charged and discharged, and electrolyzed by applying voltage to produce gas and OH—, etc. In addition, the water may decompose electrolyte solution; in particular, if the electrolyte solution is organic and fluorine is contained as an electrolyte anion therein, residual water in the electric double layer capacitor reacts with the electrolyte solution to produce hydrofluoric acid (HF), which heavily corrodes the current collectors. Furthermore, these decomposition products may cover the surfaces of the activated carbon or obstruct pores thereof, whereby the surface area thereof become smaller to cause the electrostatic capacitance to be decreased. These decomposition products also raise the resistance by covering openings of the separator. A rise in the internal resistance causes dischargeable energy to be decreased with passage of time. According to Patent Document 1, tungsten oxide contained in the electrode materials adsorb/remove water and decomposition products, whereby pores of porous carbon particles and openings of the separator are thought to be protected from covering.
Patent Document 2 provides a conductive adhesive containing a conductive material and a poly-N-vinylacetamide (PNVA)-based binder for connecting between a polarizing electrode layer and a sheet current collector, which are components of an electrode for an electric double layer capacitor. Thus the conductive adhesive has heat resistance of 250° C. or more for drying to remove water.
Patent Document 2 does not describe that this adhesive may be used as a conductive adhesive connecting between electrode materials containing tungsten oxide and a sheet current collector.